The long-term goal of this research is to understand the functional organization of the central pathways mediating the vestibulo-ocular reflex (VOR). These central pathways receive information about head velocity that has been transduced by afferents innervating the semicircular canals. Recent morphophysiological studies have shown that semicircular canal afferents can be divided into three classes: (1) low-gain, irregular afferents; (2) high-gain, irregular afferents; (3) low-gain, regular afferents There are several subclasses of secondary neurons with somas located in the vestibular nuclei that are distinguishable in terms of their discharge properties in relation to oculomotor signals. A paradigm that we have developed for silencing irregular afferents will be used to study the profile of afferent inputs received by secondary neurons. The method is based on differences in the electrical sensitivity of vestibular afferents as a function of discharge regularity. Irregular afferents are, on average, 10x more sensitive to dc currents than are regular afferents. An anodal (inhibitory) current of 100 muA is presented to both ears. This current has been shown to silence most irregular afferents to the extent that they no longer respond to rotational stimuli. The rotational sensitivity of regular afferents is not significantly affected by the currents. Squirrel monkeys are prepared for chronic recording sessions by surgically implanting a head restraining bolt, scleral search coil, recording chamber, labyrinthine stimulating electrodes, and a stimulating electrode in the rostral medial longitudinal fasciculus (MLF). The animals are trained to fixate target lights, cancel their VOR, and pursue moving targets. Single-unit activity is extracellularly recorded in the superior and medial vestibular nuclei. Once a unit is isolated, its eye position and eye velocity sensitivity are determined. The unit is tested for monosynaptic activation from the ipsilateral vestibular nerve with cathodal (excitatory) short shock stimuli and its cathodal galvanic sensitivity is recorded. The polarization paradigm is then used to measure the rotational responses in the presence and absence of 100 muA anodal currents delivered to each labyrinth independently and then to both labyrinths simultaneously. Sinusoidal chair rotations of 0.5 Hz plus or minus 400/sec and 4.0 Hz plus or minus 200/sec are used. The animal cancels its VOR during the 0.5 Hz rotations by fixating a target located straight ahead. The rotational gain of those neurons receiving irregular fiber inputs will be reduced by the anodal polarizations and the response phase will be shifted. Low- and high-gain irregular afferent inputs will be distinguished by calculating the ratio of the secondary neurons's rotational gain to its galvanic sensitivity. This ratio will be significantly lower for those units receiving predominantly low-gain irregular afferent inputs. The responses of the neuron are recorded with the animal in three different positions. Simultaneous equations are solved to determine the extent to which the neuron is receiving convergent inputs from orthogonally related semicircular canals. The neuron is then tested for antidromic activation from the MLF.